Leaching of Contaminants to Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 808

equation further allows for a broad range of transport processes (including advective
transport, diffusion, and hydrodynamic dispersion in both liquid phase and the soil
gaseous phase), while accommodating any type of chemical reaction that leads to
losses or gains in the total concentration.
Whereas the majority of contaminants in soils are usually present only in the liq-
uid phase and solid phase, and as such are transported in the vadose zone mostly
only in and by water, some contaminants such as many organic contaminants (e.g.,
VOCs, BETX), ammonium, fumigants, carbon dioxide, methane, and naturally
occurring radioactive gases including radon can have a significant portion of their
mass in the gaseous phase, and are thus also subject to transport in the gaseous
phase. The total contaminant concentration C T can be described as (Jury et al.
1991 ):

C T = ρ b s + θ c + ag (18.24)

is the volumetric water content [L 3 L − 3 ], a
is the volumetric air content [L 3 L − 3 ], and s [MM − 1 ], c [ML − 3 ], and g [ML − 3 ]are
concentrations in the solid phase, liquid phase, and soil gas, respectively. The solid
phase concentration represents contaminants sorbed onto sorption sites of the solid
phase, but can also include contaminants sorbed onto colloids attached to the solid
phase or strained by the porous system, and contaminants precipitated onto or into
the solid phase.
Various chemical or biological reactions that lead to a loss or gain of contaminant
in the soil system may be represented by the reaction term

ρ b is the bulk density [ML − 3 ], where θ

φ

of Eq. ( 18.23 ). This
includes radionuclide decay, biological degradation, and/or mineral precipitation-
dissolution. In simulation models these reactions are most commonly expressed
using zero- and/or first-order reaction rates:

φ = ρ b s μ s + θ c μ w + ag μ g − ρ b γ s − θγ w − a γ g (18.25)

where μ g are first-order degradation constants for the solid phase, liq-

uid phase, and soil gas [T − 1 ], respectively, and

μ s , μ w , and

γ s [T − 1 ], γ w [ML − 3 T − 1 ], and γ g

[ML − 3 T − 1 ] are zero-order production constants for the solid phase, liquid phase,
and soil, respectively.

18.3.1 Transport Processes

For contaminants that are present in both liquid phase and soil gas, the total con-
taminant mass flux, J T , is the result of various transport processes in both of these
phases:

J T = J l + J g (18.26)